Biomed, sometimes you don't have the clearance for the fatter couplers, and sometimes you don't want to take a chance of them coming apart and falling into the device. And sometimes, all those extensions aren't very stiff, and using them to 'thread the needle' and get into a device horizontally won't work.
I work on high end projectors and we have one adjustment that is best done with 2 @ 5mm 14" long drivers. Nothing else fits without taking lots of parts off, and you can't actually do the adjustment with the parts off...
Sometimes the right tool is the only tool.
As an aside, a simple 1/2" access hole in the case, with a cover button, would have saved 1000's of cumulative hours in the field. Of course the design was never changed.
To get back to the OP's question, I eventually found a supplier for the Bondhaus drivers that actually had them for sale. Someone online.
zuk
PS we've come an amazing distance. When I started looking, Amazon was brand new and it took some real effort. Now, you can google and find tons of online suppliers. Remember when the most info you would get from a supplier or distributor was a line card? Now you can see their stock levels at 4am from your house.
Edit link--
http://www.bondhus.com/bondhus_products/tool_categories/screwdrivers/ball.html
Has the part numbers for singles in the 'extra long' size, also at the very bottom, has the parts for customer spec'd "out of handle" length.